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Influenza Virologic Surveillance Right Size Sample Size Calculators

​The sample size calculators are designed to provide a statistical and systematic approach to estimate the number of specimens to be tested within a specific level of data confidence for situational awareness, rare/novel influenza event detection and rare/novel influenza event investigation.

​The
CDC-APHL Influenza Virologic Surveillance Right Size Roadmap defines the rationale, vital capabilities and optimal right size for influenza virologic surveillance. State and local influenza surveillance system partners are encouraged to implement a statistical, systematic approach to estimate the number of specimens to be tested in order to provide a specific level of data confidence for situational awareness, rare/novel influenza event detection and rare/novel influenza event investigation at the recommended thresholds.

Specimen sampling approaches should be established to enhance detection of rare/novel influenza events based on national thresholds, while at the same time collecting a representative sample of routine influenza cases for overall situational awareness. This website provides information and links to three sample size calculators that were designed to address the Right Size surveillance objectives. Please see the
Roadmap document for additional information on the recommended thresholds and confidence levels.

Calculator A: Situational Awareness

Determine the beginning and end of the influenza season and monitor the prevalence and spread of influenza viruses throughout the year.Go to Calculator A

How many specimens from MA-ILI patients does the laboratory need to test (in a given period, usually one week) to determine that the prevalence of Flu+ specimens among MA-ILI persons tested is x% (e.g. 10%) at a specified confidence level and error rate?Go to Calculator A

Calculator B: Rare/Novel Influenza Event Detection

Detect a rare/novel influenza virus among influenza positive surveillance specimens tested in all states at a low enough threshold for effective intervention and control measures. This objective relates to the initial detection of a rare/ novel influenza virus which generally occurs as part of routine surveillance.Go to Calculator B

How many specimens does the PHL need to test to allow the national surveillance system to detect a rare/novel influenza virus at 0.14% prevalence with 95% confidence (aggregating testing data from all states)?Go to Calculator B

Calculator B: Detecting, Monitoring Antiviral Resistance

Detect antiviral resistance virus among influenza positive surveillance specimens tested in all states at a low enough threshold for effective intervention and control measures.Go to Calculator B

How many of each influenza A subtype Flu+ or influenza B Flu+ specimens need to be tested for antiviral resistance to allow the national surveillance system to detect antiviral resistant viruses at or below a 5% prevalence with 95% confidence (aggregating testing data from all states)?
Go to Calculator B

Calculator C: Rare/Novel Influenza Event Investigation

(Rare+/ Flu+) within a state following detection of a rare/novel influenza virus (i.e. “deep dive”); confirm that the prevalence of a rare/novel influenza event does not exceed a specific percent positivity. Investigation of a rare/novel influenza event is typically performed using enhanced, targeted surveillance.Go to Calculator C

Once a rare/novel influenza virus is detected, how many ILI specimens does the PHL need to test to determine that the true prevalence does not exceed a specified percent of Flu+ within the state or in the specific jurisdiction under investigation?Go to Calculator C

Intended Audience

State and local epidemiologists, influenza surveillance coordinators, PHL directors and other senior laboratory staff responsible for coordinating policy, decisions and implementation of virologic surveillance strategies. -

Intended Use

The calculators can be used to estimate the desired number of specimens to be tested to ensure adequate confidence in influenza surveillance data and detection of rare/novel influenza viruses. Alternatively, they can be used to demonstrate the level of data confidence.